1. Field of the Invention
The present invention relates to a voltage reference circuit. More particularly, the present invention refers to a bandgap voltage reference circuit.
2. Discussion of the Related Art
In non-volatile memories with a single supply voltage it is necessary to generate different voltage reference values inside the memory that are used in the various operations of reading, writing and erasing. Some of these voltage reference values are produced by appropriate voltage regulators to which it is necessary to provide a voltage reference that remains steady over the entire range of temperature and supply that is expected.
A circuit suitable to resolve the aforesaid problem consists in a bandgap voltage reference circuit. This guarantees the stability of the reference voltage with a degree of precision in the order of a few millivolts. It is, however, necessary to design such circuit as a function of the limitations imposed by the specifications on the operation of the same. Therefore, since current technology trends impose more and more stringent specifications regarding, for instance, power consumption and start up times, it is necessary to design a bandgap voltage reference circuit that suitably meets such requirements.
The current state of the art provides the use of a bandgap voltage reference circuit in a circuit configuration whose operating principle is shown in FIG. 1. In FIG. 1 a reference current IREF flows through a bipolar transistor NPN in diode connection. The base-emitter voltage VBE of the bipolar transistor NPN is added by means of an appropriate adder node 1 with a voltage proportional to the thermal voltage       V    T    =      KT    q  
thus obtaining a voltage VBG=VBE+KVT.
Therefore it is possible to compensate the variations in temperature of the voltage VBE with the voltage proportional to the thermal voltage VT. However such compensation is realised only in a vicinity of a value of reference temperature, while neglecting the non linear terms of the base-emitter voltage. A circuit realisation with the operating principle of FIG. 1 is illustrated in FIG. 2. Such circuit realisation includes a bipolar transistor NPN Q0, in diode connection, whose emitter terminal is grounded whereas whose collector terminal is connected with the negative terminal of an operational amplifier 2 and with an end of a resistance R2. The resistance P2 has the other end connected with the output node of the operational amplifier 2 and with an end of a resistance R1. The resistance R1 has the other end connected with the positive terminal of the operational amplifier 2 and with an end of a resistance R3, that has the other end connected with the collector of a bipolar transistor NPN Q1. The bipolar transistor Q1 is in diode connection and the emitter terminal is grounded. The voltage VBG in output of the operational amplifier 2, ideal assumption, is given by the sum of a base-emitter voltage of the bipolar transistor NPN Q0 and of the voltage on the resistor R2. By taking advantage of the specifications of the ideal operational amplifier 2, a voltage VBG is obtained that is given by the following ratio:       V    BG    =            V      BEO        +                  V        T            ⁢              R1        R3            ⁢      ln      ⁢              R1A1        R2A0            
In this expression the terms RI, R2, R3 represent opportune resistances while the terms A1 and A0 are the areas of the bipolar transistors Q1 and Q0.
However, both this type of circuit configuration of the bandgap as well as the other existing circuit configurations used inside non volatile memory devices, do not allow to meet simultaneously the requirements for low power consumption and for short start up times. In fact some bandgap circuits allow to reach power consumption around 2 xcexcA but with start up times higher than 50 xcexcs while other types of bandgap circuits allow to reach short start ups times, 300 ns, but with a high power consumption, 300 xcexcA.
In view of the state of the art herein described, object of the present invention is to realise a bandgap voltage reference circuit that has both low power consumption as well as short start up times.
According to the present invention, this and other objects are attained by means of a bandgap voltage reference circuit with an output voltage that remains steady within the range of a temperature of utilization that comprises a first circuit block, a second circuit block, and a control circuit connected with said circuit blocks, said first circuit block comprising a bandgap circuit with a low power consumption, said second circuit block comprising a bandgap circuit with a short start up/start up time, said control circuit being suitable to manage said two circuit blocks in a such way that said output voltage of said bandgap voltage reference circuit is supplied by said second circuit block at the starting of said first circuit block for a period of time and said output voltage is supplied by said first circuit block for the period of time subsequent to said period of time and that lasts until the turning off of said circuit, said second circuit block being turned off after said period of time.
Owing to the present invention it is possible to realise a bandgap voltage reference circuit that has both low power consumption as well as short start up times.